1. Field of the Invention
The present invention relates to wireless communications, and in particular, to encoding uplink acknowledgments to downlink transmissions.
2. Discussion of the Related Art
One multiple carrier communication scheme transmits data through a number of orthogonal subcarriers. Examples of such systems, which typically require a high data rate, include wireless location area networks (LANs) and mobile Internet technologies. Typical multiple carrier communication schemes include orthogonal frequency division multiplexing (OFDM), discrete Fourier transform-spreading-orthogonal frequency division multiplexing (DFT-S-OFDM or DFT-Spreading-OFDM) (also referred to as SC-FDMA), and orthogonal frequency division multiplexing access (OFDMA). Although OFDM and OFDMA can achieve a high transfer rate by keeping subcarriers orthogonal, such techniques often have a high peak-to-average power ratio (PAPR). DFT-S-OFDMA is a technique which may be implemented to overcome the PAPR problem, for example. DFT-S-OFDMA functions by first spreading signals with a DFT matrix in the frequency domain before generating OFDM signals. The signals which were spread may then be modulated and transmitting in known fashion using conventional OFDM techniques. This technique will now be described.
FIG. 1 is a flowchart depicting the generation of a transmission signal according to a conventional DFT-S-OFDMA system. According to blocks 110 and 120, a typical DFT-S-OFDM wireless communication system spreads signals using a DFT matrix before generating the OFDM signals. Consider an equation in which “s” is an input data symbol, “x” is data spread in the frequency domain, and “Nb” is the number of subcarriers for a particular user. In such a scenario, the spread data “x” may be obtained using the following:x=FNb×Nbs, where FNb×Nb is a Nb×Nb DFT matrix used to spread the input data symbol.
According to blocks 130, 140, and 150, the spread vector “x” is shown mapped to a subcarrier according to a subcarrier mapping technique, and is then transformed into the time domain through an inverse discrete Fourier transform (IDFT) module to obtain a signal for transmission to a receiving entity. The transmission signal “y” may be obtained using the following:y=FN×N−1x, 
where FN×N is an N×N DFT matrix used to transform a frequency domain signal into a time domain signal. The signal “y” generated in this manner is transmitted with an inserted cyclic prefix (block 160).
Data, pilots, and control information are then transmitted in the uplink of multiple carrier systems, including, for example, the DFT-S-OFDM system. Control information can be divided into data-associated control information, which is associated with data demodulation, and non-data-associated control information, which is not associated with data demodulation.
Data-associated control information includes control information required to reconstruct data transmitted by user equipment (UE). For example, data-associated control information may include information associated with the transmit format or information associated with hybrid automatic repeat-request (HARQ). The amount of the data-associated control information can be adjusted according to an uplink data scheduling scheme.
On the other hand, non-data-associated control information is control information required for downlink transmission. For example, the non-data-associated control information may include acknowledgment (ACK) or negative acknowledgment (NACK) information for HARQ operation, and a channel quality indicator (CQI) for link adaptation of the downlink.
In an uplink multi-carrier or single-carrier FDMA system, control information is divided into a data-associated control information for demodulating user data and non-data-associated control information for downlink transmission. A basic principle of OFDM includes dividing a data stream having a high data rate into a plurality of data streams, each of which has a slow data rate, and then transmitting the data streams simultaneously using a plurality of carriers. Each the carriers is referred to as a subcarrier. Since orthogonality exists between the carriers of OFDM, if frequency components of the carriers are overlapped with each other, a transmitting terminal can still detect the frequency components.
The data stream having the high data rate is converted to a plurality of data streams having slow data rates via a serial to parallel converter. Each of the parallel-converted data streams is multiplied by a corresponding subcarrier, added together, and then transmitted to the receiving terminal.
The parallel data streams generated by the serial to parallel converter can be transmitted as a plurality of subcarriers by IDFT. IDFT can be efficiently implemented using an inverse fast Fourier transform (IFFT).
As symbol duration of the subcarrier having the slow data rate increases, relative signal dispersion, which occurs by multi-path delay spreading, decreases on the time domain. Inter-symbol interference may be reduced by inserting a guard interval longer than the channel delay spreading between OFDM symbols. If a portion of an OFDM signal is copied to the guard interval and arranged at a start portion of the symbol, the OFDM symbol is cyclically extended to be protected.
The amount of frequency resources used for data transmission may be reduced if the UE allocates a sufficient number of subcarriers to non-data-associated control information when transmitting the control information in the uplink. This technique consequently results in a large number of subcarriers which are unable to be allocated, thus affecting the ability to achieve diversity gain in the frequency domain.
A typical UE separately transmits ACK/NACK and CQI signals among non-data-associated control information in the uplink. For example, the UE transmits the ACK/NACK signal, the CQI signal, or both of these signals at a particular time period. However, conventional multiple carrier systems do not typically distinguish between such signals when processing the non-data-associated control information. This prevents efficient utilization of frequency resources.
If ACK/NACK and CQI signals are transmitted using a single discrete Fourier transform (DFT) in the uplink of the DFT-S-OFDM communication system, a number of users will typically share the same resource unit. For instance, if one user transmits an ACK/NACK signal and another user transmits a CQI signal with the same resource unit, it may not be possible for a base station to demodulate the ACK/NACK and CQI signals of the two users.